Add an import option to detect/force TextureArrays as normal maps for RGTC compression

[darthLeviN]

Describe the project you are working on

a 3d game that uses texture arrays. i want to put my normal textures in a texture array.

Describe the problem or limitation you are having in your project

compression does not support/detect normal textures. as a result when using the texture it has a z channel which is just wasted memory and bandwidth

Describe the feature / enhancement and how it helps to overcome the problem or limitation

just add a option to enable normal for texture arrays to use the dual channel RGTC compression when it's enabled.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

probably it will require mimicking some code from the regular texture importer.
or maybe just exposing the option if it's already possible in the code.

If this enhancement will not be used often, can it be worked around with a few lines of script?

no. it will require a tricky modification of the main texture array importer.

Is there a reason why this should be core and not an add-on in the asset library?

this essential is texture compression functionality.